Since insects are vectors for many diseases of humans and animals, understanding the interactions between parasites and their insect hosts is an important area of study. Circulating blood cells (hemocytes) play an essential role in defending insects against invading organisms; in large measure defining whether a given species is compatible host for a given parasite. Metazoan parasites and other large invaders are usually killed by encapsulation; a process in which certain classes of hemocytes attach and spread across the surface of the foreign target, forming a multilayered sheath of cells. Despite the fundamental importance of this defense response to invertebrates, the specific cellular and molecular mechanisms coordinating capsule formation are almost totally unknown. In this study we propose to characterize the cellular and biochemical factors that mediate capsule formation in the lepidopteran insect Pseudoplusia includens. Our background data indicate that encapsulation of foreign targets by P. includens involves specific subpopulations of hemocytes. A < 3 kDa peptide promotes certain hemocytes to change from an non-adhesive to adhesive state, while strong intercellular adhesion during capsule formation involves an Arg-Gly-Asp (RGD)-dependent adhesion mechanism. Based on these observations we propose that capsule formation requires a coordinated response by specific classes of hemocytes with adhesion mediated by an RGD-binding protein and its extracellular ligand. Specific objectives for this proposal are: 1. characterize the hemocyte subpopulations involved in capsule formation, 2. characterize the RGD- binding protein mediating cell adhesion, 3. identify the <3 kDa factor that promotes hemocyte spreading, and 4) isolate other genes involved in encapsulation. Hemocyte responses will be monitored using panels of monoclonal antibodies to specific cell populations. Characterization of factors mediating capsule formation will involve standard biochemical and molecular approaches linked to specific bioassays. Encapsulation responses are conserved across invertebrate taxa and likely involve fundamentally important mechanisms mediating cell adhesion. Thus, what we learn in this study will be relevant to many taxa and will provide important comparative information on factors regulating cell-cell communication.